The present invention relates to improved nylon 66 fiber and a process for their production. More particularly, the invention relates to nylon 66 fiber possessing a novel microstructure and improved properties such as high dye absorption, good uniformity of dyeing and good crimp performance and to a spinning process for preparing improved nylon 66 fibers involving spinning an extruded filament at a high speed.
Among polyamides fibers, nylon 66 fibers have excellent tenacity durability and stretchability and also good thermal resistance due to the high melting point, and are employed in many varied uses for apparel. On the other hand, nylon 66 fibers are poorer in uniformity of dyeing than nylon 6 fibers. For example, when nylon 66 fibers are subjected to heat processing such as false twisting, uneven dyeing easily tends to occur, compared with nylon 6 fibers, and accordingly very severe control of conditions is conducted in the steps of spinning, stretching and processing or sever production control is conducted by previously grading raw yarns before or after processing by dyeing. Such controls, however, are still not sufficient and are very disadvantageous from the viewpoint of production cost.
Known methods for improving the disadvantages of nylon 66 fibers include mixing nylon 66 with nylon 6 and copolymerizing hexamethylene adipamide with .epsilon.-caprolactam (for example, Japanese Patent Publication (unexamined) No. 72611/1976). The nylon 66 fibers produced by such methods are improved in uniformity of dyeing, but still have disadvantages such as lowering in the thermal and mechanical properties. On the other hand, it is known that the nylon 66 fibers produced by a process comprising spinning nylon 66 at a spinning speed of 3,000 m/min. to 5,000 m/min. to give pre-oriented yarns and then stretching and false twisting the pre-oriented yarns have comparatively reduced uneven dyeing. However, there are some problems such as swelling of the wound fibers, lowering in processability of the fibers and reduction in dye fastness of the textured yarns.
Generally, dyeability of polyamide fibers depends upon the concentration of terminal amino groups and the microstructure in dyeing with an acid dye or a metal complex dye or depends upon the microstructure of the fibers in dyeing with a disperse dye. Especially uniformity of dyeing of polyamide fibers is greatly influenced by the microstructure and its scattering. Further, nylon 66 fibers are denser in the microstructure than nylon 6 fibers, and the migration of dye within the nylon 66 fibers and among the nylon 66 fibers is lower and uniformity of dyeing is also inferior since the transformation of the microstructure may easily be caused by absorbed moisture due to the high capability of forming hydrogen bonds.
It is proposed that dyeability of nylon 66 fibers is improved by modifying the microstructure, but according to conventional nylon 66 fibers there is a contradictive relationship among dyeability such as dye absorbability, uniformity of dyeing and crimp performance. When one of these properties is improved, the others are deteriorated, and such nylon 66 fibers as to satisfy all these properties have not been obtained.